The invention relates to the measurement of the system transfer function of a linear, time-invariant system, such as an optical imaging system, and more particularly to a modulation transfer function test compensation for variations in test pattern duty cycle.
Refer now to FIG. 1 which shows a square wave as a function of time. The system transfer function, STF, of an optical imaging system is composed of both the modulation transfer function, MTF, and the phase transfer function, PTF. Optical test patterns and an image analysis computer program have been used to measure the STF. For example, one such system is described in U.S. Pat. No. 5,621,519, entitled "Imaging System Transfer Function Control Method and Apparatus," issued Apr. 15, 1997 to Frost et al. that is incorporated by reference hereto. In Frost et al. the optical test pattern apparatus is intended for use with a microscope of high magnification and employs a test pattern with test pattern geometry having very small dimensions. The test pattern geometry includes bar patterns. These bar patterns have a duty cycle. An ideal square wave is a function that can take on only two values. FIG. 1 illustrates such a function. The duty cycle is defined as the ratio of the length of time in a single cycle for which the function's value is at the high level divided by the period of the function. FIG. 1 shows a duty cycle equal to W divided by T. An example ideal bar pattern duty cycle is 50%, or half of the pattern being light and half of the pattern being dark.
The required small dimension test pattern geometry makes it difficult to manufacture the test pattern. The limitations of the test pattern fabrication preclude the production of a large number of test patterns with a bar pattern duty cycle precisely equal to 50%. Errors in the bar pattern duty cycle cause errors in the MTF measurement and in the STF measurement. The current invention provides a method for compensating for errors in the test pattern duty cycle, thus restoring the accuracy of the MTF measurement when a test pattern with duty cycle other than 50% is used.